1. Field of the Invention
The present invention relates to a circuit breaker with a parallel resistor and, in particular, relates to a circuit breaker with a parallel resistor which in the parallel resistor is used in common for a resistance circuit making contact and a resistance circuit breaking contact connected in parallel with respect to a main contact for reducing a switching surge or a transient recovery voltage during switching.
2. Description of Related Art
Circuit breakers with a parallel resistor, such as a gas circuit breaker with a circuit making resistor in which the circuit making resistor for suppressing a circuit making surge is connected in parallel with a main circuit breaking portion and an air circuit breaker with a circuit breaking resistor in which the circuit breaking resistor for reducing a transient recovery voltage rising rate during the current interruption is connected in parallel with a main circuit breaking portion, are generally known, and circuit breakers incorporating both the circuit making resistor and the circuit breaking resistor are comparatively a few.
However, in a power transmission system having a very high operating voltage such as UHV power transmission system of 1100 kV level, it is necessary to suppress the switching surge level at a low level for reducing the construction cost thereof and it is considered essential for gas circuit breakers used in such power transmission system to employ both the circuit making resistor and the circuit breaking resistor.
As one example of these types of circuit breakers, a structure of the circuit breaking portion is known as disclosed in JP(U)-A-57-185145(1982) of which an equivalent circuit is illustrated in FIG. 2. The circuit breaker is constituted by a main circuit breaking portion including a separable main contact 1, a resistance circuit making portion electrically connected in parallel with the main contact 1 and including an open and closeable resistance circuit making contact 2 and a circuit making resistor 4 and further a resistance circuit breaking portion electrically connected in parallel with the main contact 1 and including a separable resistance circuit breaking contact 3 and a circuit breaking resistor 5. Characteristics of the circuit breaker with regard to the opening and closing operation are shown in FIG. 3 wherein the characteristics of contact separating operations S1, S2 and S3 for the main contact 1, the resistance circuit making contact 2 and the resistance circuit breaking contact 3 are illustrated along with passage of time. In the circuit breaking operation "O", the resistance circuit making contact 2 begins to separate at time t1 prior to the separation of the main contact 1 at time t2, via the separation of the main contact 1 at time t2 the circuit breaking current is shifted to the circuit breaking resistor 5, at time t3 after several tens ms from the separation of the main contact 1 the resistance circuit breaking contact 3 is separated to undergo the current interruption at the resistance circuit breaking portion. Further, in the circuit making operation "C" (indicating "closing") at time t4 several tens ms prior to time t5 when the main contact 1 is made the resistance circuit making contact 2 is made to permit a predischarge current to flow through the circuit making resistor 4 and the resistance circuit breaking contact 3 is made at the same time t5 with the main contact 1 or at time t6 several tens ms thereafter.
Other than the above circuit breaker in which the main circuit breaking portion, the resistance circuit making portion and the resistance circuit breaking portion are separately constituted, JP-A-56-11816(1981) discloses a circuit breaker in which the resistance circuit making portion is constituted so as to serve as the resistance circuit breaking portion of which equivalent circuit is illustrated in FIG. 4, wherein in electrically parallel with the main contact 1 a series connection of a resistance contact 6 and a resistor 7 is connected and the opening and closing operation characteristics S1 and S6 of the both contacts 1 and 6 are illustrated in FIG. 5. Namely, in the circuit breaking operation "O"(indicating "opening"), after the main contact 1 is opened at time t2, at time t3 several tens ms thereafter the resistance contact 6 is opened to interrupt the current flowing through the resistor 7. On the other hand, in the circuit making operation "C"(indicating "closing"), at time t4 about 10 ms prior to time t5 when the main contact 1 closes, the circuit making contact 6 closes to insert previously the resistor 7 into the circuit. As a result, the resistor 7 and the resistance contact 6 perform both functions of the resistance circuit making portion and the resistance circuit breaking portion.
The conventional circuit breakers with a parallel resistor are constituted as explained above. Therefore in case of the circuit breaker as shown in FIG. 2, certain voltages appear at the terminals of the both resistors 4 and 5 on the sides of both contacts 2 and 3 when currents flow through the both resistors 4 and 5 and the insulation for the both voltages appearing at the terminals has to be maintained in such a manner that in the vicinity thereof conductors have to be disposed with predetermined spaced apart dielectric distances l1 and l2. These dielectric distances l1 and l2 for a high voltage circuit breaker reach upto several tens.about.several hundreds mm which causes a size increase of the circuit breaker. Further, during the circuit breaking operation "O", since the current interrupting capacity of the resistance circuit making contact 2 is low the resistance circuit making contact 2 has to be separated prior to the separation of the main contact 1, and the current to be interrupted has to be controlled to flow only through the main contact 1 and not through the resistance circuit making contact 2. For this reason the dielectric recovery voltage characteristics between the electrodes of the resistance circuit making contact 2 during its circuit opening operation always has to exceed those of the main contact 1. On the other hand, during the circuit making operation "C", a prior discharge always has to be caused at the resistance circuit making contact 2 to insert the resistor 4 into the circuit. The requirement during the circuit making operation "C" for the resistance circuit making contact 2 in which the electrode structure has to be designed likely to cause the prearcing is incompatible with the requirement during the circuit breaking operation "O" in which the electrode structure has to be designed to relax sufficiently the electric field caused thereby for obtaining a high dielectric recovery voltage characteristics between the electrodes. The same problem arises with respect to the circuit breaker shown in FIG. 4. Namely, the resistance contact 6 has to interrupt the current at the last time during the circuit breaking operation "O", therefore the electrode structure has to be designed to sufficiently relax the electric field therearound so as to withstand a high recovery voltage which will appear between the electrodes. On the other hand, the electrode structure during the circuit making operation "C" always has to be designed to initiate the prearcing and to insert the resistor 7 into the circuit, and the circuit breaker shown in FIG. 4 likely has to fulfill the incompatible two requirements.